Electric Chainsaw Mill with load regulation

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wileur

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Hi,

I'm in the middle of building an electrical chainsaw mill. Since I have a background in industrial automation, electronics development and embedded programming I decided to automate things quite a bit.
I'm going to use a 5.5kW three-phase motor to power the saw, a smaller motor for feeding and and a 24V geared DC-motor with wires and pulleys to lift the saw head. An optical rotary encoder will help keep track of the height.
The bigger motors will be controlled by two frequency drives I had laying around. I'll put some inductive sensors to act as limit switches to automatically turn of the saw and feeding when reaching the end of the log.
The rails and log bed will somewhat resemble a Norwood PM14, but the saw carriage will be different as I'll be using parts of a pallet wrapping machine.
Most of the electronics has been completed while waiting for other parts to arrive and I have welded most of the motor mount.
Instead of a very expensive 6m drag chain I'm thinking to tie several cables together to form a stable flat cable.

Now to my question/thought: Would it make sense to adjust the feed rate based on the main saw motor load? Since I use VFDs and modbus communication I can easily monitor current, power draw or rpm and should be able to continuously adjust the feed rate to accommodate for varying log thickness, dryness, density, knots etc. If the saw starts to bog down, the system will slow down the feed a bit. Likewise, if it cuts like through butter it can move faster.
From my experience in using an alaskan mill this sounds like a good idea, but I might be overthinking it.


Best regards,
Wileur
 

GrizG

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From my 18th century gun barrel reaming experience: pulling the saw through the log with a weighted cable would do the job... on a wide log, or a hard one, or one with knots, the feed would slow down without any attention. See the video at about the 5 minute point for an example of an associate of mine using the technique... We had a different but similar reaming machine when I worked there.

Mixing some 18th century tech with some 21st century tech would be an interesting contrast. 😉
 

wileur

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From my 18th century gun barrel reaming experience: pulling the saw through the log with a weighted cable would do the job... on a wide log, or a hard one, or one with knots, the feed would slow down without any attention. See the video at about the 5 minute point for an example of an associate of mine using the technique... We had a different but similar reaming machine when I worked there.

Mixing some 18th century tech with some 21st century tech would be an interesting contrast. 😉

Interesting video! The weight and rope idea is good and I saw a guy on YouTube use it with his Alaskan mill and it worked well.
It was my original plan too, but I like automation so I added a motor for feeding the carriage.

My takeaway then is that constant pressure is worth the effort, so I'll look more into that.
Thanks!
 

lwmibc

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My experience with a carriage-mounted CSM also supports the idea of load-sensing variable carriage speed, I push the carriage by hand deliberately so I can maintain a constant load and consistent chain speed. It seems to give me a cleaner and more consistent surface.
 

GrizG

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Interesting video! The weight and rope idea is good and I saw a guy on YouTube use it with his Alaskan mill and it worked well.
It was my original plan too, but I like automation so I added a motor for feeding the carriage.

My takeaway then is that constant pressure is worth the effort, so I'll look more into that.
Thanks!
Ceteris paribus, consistent pressure via a variable feed rate captures the notion a bit more fully... I'd think that constant pressure alone could overwhelm the saw's ability to cut some times and leave it operating at below capabilities levels at others...
 
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A load cell rigged to engine mounts on a simple pivot or something RPM related as a speed controller on the feed rate was my best idea. Not sure if it's practical. Load cells are expensive if you buy stuff Baha1000 race ready quality parts. This is how a modern water brake dyno works. Calculate the torque and get all the info. Maybe watch your exhaust gas temps also. Another way to control load based on head temps.
 

wileur

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Ceteris paribus, consistent pressure via a variable feed rate captures the notion a bit more fully... I'd think that constant pressure alone could overwhelm the saw's ability to cut some times and leave it operating at below capabilities levels at others...
Yes, you are right. I was thinking more along the lines of consistent load on the motor, by varying the feed rate.

A load cell rigged to engine mounts on a simple pivot or something RPM related as a speed controller on the feed rate was my best idea. Not sure if it's practical. Load cells are expensive if you buy stuff Baha1000 race ready quality parts. This is how a modern water brake dyno works. Calculate the torque and get all the info. Maybe watch your exhaust gas temps also. Another way to control load based on head temps.
I believe I have a pretty good way of monitoring the load. Since I'm using an electric motor with a Variable Frequency Drive with Modbus communication to my controller I can continuously read the load power or current.
 

wileur

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A few random pics of the project.
 

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Yes, you are right. I was thinking more along the lines of consistent load on the motor, by varying the feed rate.


I believe I have a pretty good way of monitoring the load. Since I'm using an electric motor with a Variable Frequency Drive with Modbus communication to my controller I can continuously read the load power or current.
That is reading the saw head combined with drive effort not one or the other.
 

wileur

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@Lightning Performance:
I might be missing something here, but isn't it enough to read the saw head load? Why drive effort?

As I wrote above, I'm using three-phase 400V asynchronous motors for the saw head and feed, controlled by sensorless vector VFDs. My understanding is that if I tell the VFD to run the motor at its nominal frequency of 50Hz, it will try to maintain this speed under varying loads by pushing more or less power into the motor.
My assumption is that motor load is basically affected by wood hardness and feed rate. Of these I can control the feed rate.
(Chain condition might also affect things, but I'll change/sharpen the chain when I get saw dust instead of chips.)

The feed motor is also controlled by a VFD, which strives to keep the feed constant at the rate my system sets. So as long as I have enough power headroom I consider the feed rate to be an output variable, instead of something to be measured.


So, in short, both VFDs+motors will automatically try to stay at their set points, and I'll adjust the feed rate set point based on the motor load reading.


If this is still the wrong way to go about it, please let me know. I'm happy to improve and learn.
 
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I realize after considering my reply that the system actually does what you suggest. The feed drive effort is managed directly by the control loop between the VFD and motor.

Thanks for making me think!
It's just like you're driving it with both controllers.
You got this pretty much figured out.
I'm thinking it's worth copying. If you're nearby and something does go wrong no big deal. The load cell can be used as fail safe kill switch for a full stop.
 
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